The present invention is directed to methods and apparatus for wastewater treatment, and more particularly, is directed to sequential batch reaction methods and apparatus for wastewater treatment.
Wastewater treatment and treated effluent goals and standards have become increasingly stringent for the economical removal of wastewater components such as total suspended solids (TSS), biological oxygen demand (BOD), nitrogen (as nitrate and ammonia) and phosphorous from large volumes of municipal and industrial wastewater. Activated sludge systems of either the continuous flow type in which an influent stream is continuously treated and continuously discharged through one or more treatment zones, or the sequencing batch reactor type in which a continuous influent stream is sequentially treated and intermittently discharged, are conventionally used for wastewater treatment. In such activated sludge treatment systems, treatment microorganisms are concentrated in the treatment system in order to more rapidly remove the wastewater impurities, including BOD, nitrogenous, and phosphorous components of the wastewater. The highly diverse, mixed cultures utilized in such activated sludge wastewater treatment systems for biological removal of BOD, nitrogen and phosphorous include ordinary heterotrophs (which can consume organic wastewater components to produce carbon dioxide and reduce BOD, as well as mediate denitrification), autotrophs (which mediate nitrification in consuming nitrogenous wastewater components) and phosphotrophs (which can accumulate polyphosphates in consuming phosphorous-containing wastewater components).
The various types of microorganisms in activated sludge cultures typically utilize different nutrient, oxygenation and other conditions for optimum removal of different wastewater components. The organic materials in the wastewater are consumed by xe2x80x9cactivated sludgexe2x80x9d microorganisms for both energy and cell synthesis, driven by biological oxidation-reduction reactions involving transfer of electrons from a wastewater component to be oxidized (the electron donor) to an oxidizing material (the electron acceptor). Heterotrophic metabolism utilizes organic wastewater components as electron donors, while autotrophic metabolism utilizes inorganic wastewater components as electron donors. In aerobic systems in which the wastewater is aerated, oxygen is utilized by xe2x80x9cactivated sludgexe2x80x9d microorganisms as the terminal electron acceptor. In anoxic systems, the oxygen is substantially depleted, and xe2x80x9cactivated sludgexe2x80x9d microorganisms utilize nitrates and nitrites as the primary terminal electron acceptors. Under anaerobic conditions, oxygen, nitrate and nitrite components are substantially depleted, and carbonates and sulfates serve as primary terminal electron acceptors in the cell reactions (M. G. Mandt and B. A. Bell xe2x80x9cOxidation Ditchesxe2x80x9d, 169 pgs., 1982, Ann Arbor Science Publishers). It should be noted that different microorganisms and/or metabolic pathways may predominate under such different aerobic, anoxic and anaerobic conditions.
Sequencing batch reactors such as described in U.S. Pat. No. 4,596,658 to Mandt, are conventionally utilized for wastewater treatment to provide high quality effluent by subjecting a given volume of wastewater to a predetermined sequence of different treatment steps in batch mode, in the same batch reactor equipment. In this regard, a volume of wastewater may typically be introduced as a continuous or discontinuous feed stream into a sequencing batch reactor treatment system and subjected to extensive mixing and aeration for a predetermined period of time to provide biological oxidation, consumption or other removal of wastewater components. The mixing and aeration may subsequently be stopped and the wastewater maintained in a quiescent state in the same treatment zone to permit wastewater solids, including microbiological treatment organisms, to settle in the reactor. A clarified portion of the treated wastewater may be subsequently removed from the upper portion of the reactor, which in turn may be conducted to subsequent treatment and discharge steps. Additional wastewater which is to be treated may then be introduced into the sequencing batch reactor, and the cycle repeated. For many wastewater treatment applications, sequencing batch reactors may provide a number of advantages over older type continuous flow treatment systems in terms of expense, physical area and operating energy requirements. However, although sequencing batch reactors have proven to be efficient, flexible and economic wastewater treatment systems, further improvements which could increase the processing efficiency, and/or optimize treatment conditions, such as anoxic and aerobic treatment conditions, for wastewater component removal would be desirable. Such improved sequencing batch reactor methods and apparatus would be desirable which would be simple and effective in operation, which would permit enhancement and synergistic interaction of anoxic and aerobic treatment conditions for assisting wastewater component removal, and which would enhance the utility and cost effectiveness of sequencing batch reactors for wastewater treatment.
Accordingly, it is an object of the present invention to provide such improved methods and apparatus and sequencing batch reactor systems which utilize such methods and apparatus.
In many biological treatment plants treating municipal wastewater, approximately 1 to 2% of the influent by volume exits the treatment process as dilute waste sludge (WAS) requiring further treatment and/or disposal. The further treatment and disposal of this 1 to 2% dilute waste sludge may represent a significant part (e.g., up to 50%) of the total cost of wastewater treatment in a modern treatment plant. In addition to the capital costs for tankage and equipment for sludge reduction, dewatering, hauling, and ultimate disposal, there are significant continuing operating costs for power, treatment chemicals, hauling and landfill fees. The continuing operating costs for sludge reduction, dewatering, hauling and ultimate sludge disposal may even constitute the most substantial portion of the cost in municipal wastewater operating budgets. Furthermore, these costs have tended to increase in recent years with increasing public and political opposition to hauling and disposal of sludge in many localities, thereby limiting disposal sites and capacities.
Many conventional municipal wastewater treatment plants process waste sludge by using anaerobic or aerobic digestion for pathogen and organic sludge reduction in the waste sludge produced by suspended growth biological wastewater treatment systems, such as the various continuous flow activated sludge systems, sequencing batch reactor systems, and fixed growth biological systems including trickling filters or rotating biological contactors. Regardless of the source, the waste sludge (WAS) is typically dilute, generally less than 1-2% solids content by weight. The total suspended solids (TSS) contained in such sludge consists of organic or volatile suspended solids (VSS) and inorganic, inert or fixed suspended solids (FSS). The organic fraction is typically about 70% of the total suspended solids and comprises microorganisms, cellulose, bits and pieces of plastic, and other insoluble organic compounds. Depending on influent constituents and the type of biotreatment system used to treat the sewage, VSS will typically range from about 60% to 90% of TSS. Most larger wastewater treatment plants, and substantially all small and medium size wastewater treatment plants, use aerobic sludge digestion rather than the more complex anaerobic digestion. In aerobic digestion, the waste sludge is held in a tank or tanks where it is repetitively aerated and thickened by gravity settling and decanting of supernatant. The supernatant may be recycled to the sewage processing biotreatment plant. The remaining digested sludge is highly hydroscopic, and as a practical limit generally cannot readily be thickened beyond 2-3% by weight solids concentration.
The United States Environmental Protection Agency (the EPA) recommends that the waste sludge be held and aerated long enough to destroy 38% of the VSS conetnt in order to reduce pathogens and odor potential of the sludge, and to produce a more stable sludge which is suitable for liquid hauling and land disposal or further dewatering and processing. Dewatering may be accomplished by chemical treatment using relatively large doses of expensive, synthetic polymers to counteract the hydroscopic nature of the sludge, agglomerate the solids and allow further water separation. Horizontal, solid-bowl centrifuges or belt filter presses are typically used to mechanically separate water from the polymer-treated sludge, increasing solids content of the sludge to typically 15 to 25% by weight. At this point, the sludge is truckable and can be hauled to a landfill. Alternatively, sludge drying and incineration or composting have been used to further process the sludge to reduce its volume.
Achieving the U.S. EPA-recommended 38% reduction of VSS by aerobic digestion typically requires considerable tankage, as well as extended aeration contact or retention time. Tankage requirements may be, for example, about 25% to 50% of the tankage volume for the main sewage treatment system. In this regard, a plant treating 1 million gallons per day (MGD) of municipal sewage containing 200 mg/l of BOD5 and 200 mg/g TSS in the influent may produce about 1700 pounds per day of waste sludge. If the sludge is removed or xe2x80x9cwastedxe2x80x9d at 1% solids content, roughly 20,000 gallons per day (gpd) of waste sludge must be wasted from the treatment plant, which amounts to approximately 2% of the influent flow. Assuming 30 days sludge holding time is required for the aerobic digestion of the sludge to insure removal of at least 38% of the sludge VSS, the required aerobic digestion tankage of 600,000 gallons may approach or equal the tankage requirements for the actual sewage treatment. Some states such as Iowa, which prohibit land application in winter when the ground is frozen, require 180 days of sludge storage, which significantly increases the tankage requirements. In this example, of the 1700 pounds per day of sludge requiring aerobic digestion, roughly 70%, or 1190 pounds, may be organic (VSS), leaving 510 pounds of inorganics or biologically inert materials which cannot be biologically oxidized. If 38% of the VSS is consumed or destroyed, there will still be roughly 738 pounds of VSS in the sludge. The digested sludge at that point will be roughly 60% organic and 40% inorganic. Sludge is digested and consumed (destroyed) by biological oxidation of organics and auto oxidation of microbial biomass. If digested sludge leaves the digester at 1.5% solids content, roughly 10,000 gallons per day, or 1% of the influent wastewater to the treatment plant, must be wet hauled to land disposal or sent to further processing. Accordingly, it is an objective of some embodiments of the present disclosure to provide treatment systems which can substantially reduce the amount of sludge which must be disposed of by landfill or further processing.
Other objectives of various optional embodiments of the present disclosure are to provide treatment systems and processes which contain surface scum and quiescently transfer wastewater, and/or which are capable of reducing the total amount of sludge for disposal from about 1% or more to less than 0.011% by volume of influent wastewater to be treated. A further objective of such embodiments is to produce a stable, relatively inert byproduct having improved, xe2x80x9cless sludge-likexe2x80x9d characteristics for ultimate disposal on site or in local landfills.
These and other objects of the disclosure (which may each be independent of other objectives in different embodiments of the invention, or may be combined with other objectives, particularly in preferred embodiments), will become more apparent from the following detailed description and the accompanying drawings.